Parallel Computing Toolbox™

spmd - Execute code in parallel on MATLAB pool

Syntax

spmd, statements, end
spmd(n), statements, end
spmd(m, n), statements, end

Description

The general form of an spmd (single program, multiple data) statement is:

spmd
    statements
end

spmd, statements, end defines an spmd statement on a single line. MATLAB executes the spmd body denoted by statements on several MATLAB workers simultaneously. The spmd statement can be used only if you have Parallel Computing Toolbox. To execute the statements in parallel, you must first open a pool of MATLAB workers using matlabpool.

Inside the body of the spmd statement, each MATLAB worker has a unique value of labindex, while numlabs denotes the total number of workers executing the block in parallel. Within the body of the spmd statement, communication functions for parallel jobs (such as labSend and labReceive) can transfer data between the workers.

Values returning from the body of an spmd statement are converted to Composite objects on the MATLAB client. A Composite object contains references to the values stored on the remote MATLAB workers, and those values can be retrieved using cell-array indexing. The actual data on the workers remains available on the workers for subsequent spmd execution, so long as the Composite exists on the client and the MATLAB pool remains open.

By default, MATLAB uses as many workers as it finds available in the pool. When there are no MATLAB workers available, MATLAB executes the block body locally and creates Composite objects as necessary.

spmd(n), statements, end uses n to specify the exact number of MATLAB workers to evaluate statements, provided that n workers are available from the MATLAB pool. If there are not enough workers available, an error is thrown. If n is zero, MATLAB executes the block body locally and creates Composite objects, the same as if there is no pool available.

spmd(m, n), statements, end uses a minimum of m and a maximum of n workers to evaluate statements. If there are not enough workers available, an error is thrown. m can be zero, which allows the block to run locally if no workers are available.

Examples

Perform a simple calculation in parallel, and plot the results:

matlabpool(3)
spmd
  % build magic squares in parallel
  q = magic(labindex + 2);
end
for ii=1:length(q)
  % plot each magic square
  figure, imagesc(q{ii});
end
matlabpool close

See Also

batch, Composite, labindex, matlabpool, numlabs, parfor

speye

sprand